1. Field of the Invention
Embodiments of the invention relate to optical devices, and more particularly to optical devices including digital imagers.
2. Description of the Related Art
Optical devices suffer from a position-dependent light intensity variation termed “vignetting.” Vignetting refers to an effect where an image is brighter towards the center of the image and fades towards the edges thereof. Vignetting is typically caused by characteristics of optical lenses of an optical device, which results in an uneven distribution of light intensity on the optical field (e.g., film or image sensor array) of the optical device.
Digital optical devices also suffer from vignetting. Digital optical devices typically include a lens system and an image sensor (or imager) that digitally captures images. Vignetting is particularly acute in low-cost digital optical devices that use an inexpensive optical lens, and have a relatively short distance between the lens and the image sensor thereof. When digital optical devices are in use, pixel data (e.g., luminance data) of a captured image can be processed to restore the brightness of the image, thereby compensating for vignetting in the digital optical devices. For example, pixel data can be multiplied by position-dependent gains to restore the brightness to the correct level. In some arrangements, look-up tables are employed to provide position-dependent gains across an array of pixels. However, such look-up tables need a large amount of memory, which increases the manufacturing costs.
To minimize memory requirements, pixel correction functions have been used for compensating for vignetting. Pixel correction functions typically provide position-dependent gains such that pixel data from pixels near edges of an image sensor array are multiplied by a larger correction gain than data from pixels near the center of the array. Various functions have been proposed as pixel correction functions, including polynomial functions, exponential functions, trigonometric functions, and hyperbolic cosine functions. Such pixel correction functions need to be calibrated for the position of the optical lens relative to the image sensor array in each individual digital optical device. The calibration process can be performed for individual digital optical devices during or after the manufacturing process. It would therefore be advantageous to provide a simplified calibration process for digital optical devices.